For many years, attempts have been made to develop gasket materials and configurations which provide an effective seal between two adjoining surfaces under a wide range of temperatures and pressures and a variety of environmental conditions. Initially, attempts were made to develop gasket configurations having both a high resistance to relaxation or compression set and high deformability and compressibility characteristics. In pursuit of these goals, various composite gasket structures were developed in an attempt to exploit the desirable characteristics of different materials incorporated in the composite gasket. For example, U.S. Pat. No. 2,584,959 to A.M. Yocom et al discloses a cork containing composite gasket material which incorporates a layer of asbestos-rubber and one or more layers of cork. Rubber-asbestos has a high resistance to relaxation or compression-set but lacks conforability unless it is subjected to high pressures. Cork, on the other hand, exhibits high conformability at low pressures but also has a low resistance to relaxation or compression set.
In recent years, opposition has been raised to the use of asbestos as a gasket material and, consequently, manufacturers began experimenting with alternative constructions in an effort to replace or eliminate asbestos. The Jonnes et al U.S. Pat. No. 3,524,794 illustrates a composite fluid sealing gasket designed to provide both high conformability under pressure conditions and a high resistance to relaxation or compression-set. This gasket includes a layer formed of hollow, rigid, collapsible particles which are fully embedded in an elastomeric binder. The particles are hollow glassy spheroids (microbubbles) which collapse in response to pressure. Two layers of elastomeric binder with embedded microbubbles may be bonded to a central, strength-imparting film, which may be elastomeric or non-elastomeric, to form a laminate.
The prior art composite gasket assemblies illustrated by these patents operate effectively for many normal gasket applications, but such gaskets are not adapted for use in severe or corrosive operating conditions. For example, many chemical applications require gasketing material which will not deteriorate when subjected to chemicals which range from mildly to severely corrosive and the same gasketing material must also be resistant to attack by aggressive atmospheres where wide temperature ranges of from -350.degree. F. to +500.degree. F. may occur. In addition to withstanding severe operating conditions, such gasketing material must also meet the double requirement of being highly compressible and deformable to assure a tight, lasting seal in response to low flange pressures. For many applications where corrosive chemicals are present, glass lined conduits having glass lined flanges are provided, and the bolt-load pressure on the flanges must be low to avoid damaging the glass lining for the flange.
Recently, gasketing material containing polytetrafluoroethylene (PTFE) has been used in corrosive chemical environments. PTFE is inherently tough, has excellent chemical resistance and good tensile strength, and will withstand a wide range of temperatures. Although PTFE is chemically inert, it does not exhibit exceptionally high compressibility in response to low flange loading pressures and consequently, composite gasketing assemblies have been developed in an attempt to use the chemical resistant properties of PTFE while still enhancing gasket compressibility. Thus, envelope gaskets were developed wherein an outer envelope of PTFE was formed and was then filled with a more compressibile filler material such as compressed asbestos or other felted gasket filler. The PTFE jackets for the envelope gaskets provide chemical resistance while deformability is provided by the filler material.
Unfortunately, envelope gaskets are subject to a number of disadvantages. The envelope jacket often will fold over on itself during installation of the gasket, thereby creating creases in the gasket that cause leaks. Also, there may be pin hole leaks in the envelope itself, causing caustic material to attack the envelope filler from outside resulting in degradation of the filler. In some instances, the envelope jacket of PTFE will separate from the deformable filler material and ripples or folds may occur merely from stretching the envelope over the filler. Also, if uneven flange torquing occurs, the jacket may stress or burst, and envelope gaskets are subject to cold flow or creep which requires periodic bolt retorquing.
A major disadvantage of envelope gaskets is that the least compressible component, namely, the PTFE envelope is outermost, and this element is not highly deformable under low flange pressure loads. Too often, when a glass lined flange is torqued sufficiently to cause an envelope gasket to provide an effective seal, destruction of the glass lining for the flange results.
In an attempt to rectify some of the problems associated with envelope seals, a homogeneous PTFE gasketing material filled with microbubbles; i.e., glass microballoons, was developed. This material, as illustrated by Garlock Style 3504 gasketing manufactured by Garlock, Inc. of Palmyra, New York, uses glass microballoons to impart compressibility (25% to 35%) to a PTFE binder, thereby providing a more deformable gasket without the disadvantages experienced by multiple component gaskets. This homogeneous PTFE gasketing material exhibits enhanced compressibility and sealing characteristics due to the incorporation of microballoons, while maintaining the resistance to chemicals and the enhanced temperature characteristics provided by PTFE. However, the addition of the microballoons to the PTFE lowers the tensile strength properties which would be provided by pure PTFE gasketing.